A process of low-temperature fractionation and a corresponding apparatus are disclosed by EP 810412A. There, the nitrogen-rich fraction is recompressed by means of a compressor prior to its mixing.
Another process is disclosed by DE-38 14 187-C2. Here, impure nitrogen from an intermediate point from the low-pressure column is recycled upstream of the first stage of the air compressor. Still another process is shown in U.S. Pat. No. 4,848,996, where the impure nitrogen is taken off at the top of the low-pressure column and the feed air is added at an intermediate stage of the air compressor.
Recycling the nitrogen-rich fraction into the feed air is advantageous per se and increases the product yield. Nevertheless, the process can be improved.